The present invention relates to a fixing device, a web differential gear and an image formation apparatus using the fixing device and the web differential gear. More specifically, it relates to an offset preventing structure in a fixing step.
In image formation apparatuses such as copying machines, facsimile, printers and printing machines, there are those having a configuration using an electrophotography method as an image formation method. According to the electrophotography method, a visualizing process is executed for an electrostatic latent image supported on a photosensitive member as the latent image supporting member using a developing agent such as a toner.
As the developing agent used for the visualizing process, a one component type developing agent with a toner itself provided with the charging property and the magnetic property so as to enable conveyance or electrostatic adhesion to the electrostatic latent image and a two component type developing agent including a toner and a carrier are included. Recently, in order to meet the demand for the image quality improvement, a small particle size has been adopted in the toners. By using a small particle size toner, the image quality can be improved by improving the dot reproductivity or the sharpness.
According to an image formation apparatus using the electrophotography method, when the transfer step for the toner image as the visual image supported on the photosensitive member as the latent image supporting member is finished, a fixing step is executed for the sheet with the toner image transferred. In the fixing step, for example, a configuration utilizing a thermal roller fixing method having a fixing roller with a heat source stored and a pressuring roller with an elastic layer on the surface facing with each other in a sheet conveyance path for heating and fusing a toner image so as to be permeated in the sheet can be presented. In addition to the thermal roller fixing method, a fixing method using a belt, a fixing method utilizing a film, as to the heating source, in addition to the heater, a method for heating by induction heating, or the like are discussed.
According to the fixing device, the offset phenomenon has been known for a long time, and various countermeasures have been provided therefore. As one of them, a technique of eliminating the offset toner adhered on the fixing member by cleaning and preventing re-adhesion has been proposed. As an embodiment for cleaning, one comprising a web cleaning device which uses a web has been provided.
The web is made of a non-woven fabric produced as a mixture of fibers of an aramide and a polyethylene terephthalate, is soaked in an oil as needed for eliminating the toner or coating the oil while sliding on the fixing roller surface.
However, recently, for the purpose of improvement of the reproductivity and the sharpness, a fixing device which fixes a toner image formed using a small particle size toner has been developed. When a toner formed using a small particle size toner is fixed, it is confirmed that the amount of the offset toner is extremely large compared with the case of a toner of an ordinary particle size (about 20 xcexcm or more as the volume average particle size).
In order to find the cause thereof, the present inventor has discussed various aspects so as to have the following assumption for the reason. The toner image using a small particle size toner (about 5 xcexcm to 10 xcexcm volume average particle size (average value of the particle size of the toner particles contained in a unit volume)) is fused and permeated while being held and conveyed between the fixing roller and the pressuring roller as in the case of a toner image using a non-small size particle size toner (about 20 xcexcm xcexcor more volume average particle size conventionally used).
In the sheet with the toner image supported, particularly in a paper such as a transfer paper, ruggedness exists on the surface so that when a toner enters in the ruggedness, contact of the fixing roller and the toner is insufficient so that heat cannot be supplied to the toner so as to have the unfixed toner. As a result, there is a risk of fixation of the unfixed toner with insufficient fusion and permeation can be offset on the fixing roller surface.
FIGS. 6A and 6B are diagrams which explain the phenomenon. The toner supported on a sheet S passing through a fixing nipper comprising a fixing roller A and a pressuring roller B shown in FIG. 6A has insufficient contact with the fixing roller A when it is entered in the ruggedness of the sheet, in particular, in the recess part with a wider facing interval with respect to the fixing roller A as shown in FIG. 6B.
Particularly in the case of a small particle size toner, the amount entered in the recess part is large so that the toner in the unfixed state due to the insufficient contact is large as well, and thus the toner amount generating the offset due to the counter transfer to the fixing roller is large as well. An offset preventing layer using a mold releasing agent such as a fluorine resin is formed on the fixing roller A surface for the offset prevention. Depending on the material used for the offset preventing layer such as the fluorine resin, cracking, holes or grooves may be generated as time passes by so that the surface can be rugged. Since a small particle size toner of a less than 5 xcexcm size is used and the size of the cracking, holes or grooves in the offset preventing layer is about 4 to 5 xcexcm, the unfixed toner can easily enter on the sheet.
From the viewpoint of the environment conservation, papers once used for image formation are reused increasingly at the offices. In the case of such papers, ruggedness exists on the surface due to the toner image so that the surface evenness is poorer than an ordinary paper. Therefore, offset can easily be generated in this kind of papers.
As heretofore explained, although a small particle size toner (about 5 xcexcm to 10 xcexcm volume average particle size) provides a higher image quality, it also involves a disadvantage of the offset toner increase. Therefore, at the time of using a small particle size toner, in order to reduce particle size irregularity, a work for further eliminating a small particle size toner is executed in the refining step (one with a less than 5 xcexcm particle size, hereinafter referred to as fine powdery toner). In order to restrain the influence on the image formation apparatus, the fine powdery toner is adjusted to be about 10 number % as a whole (it referred to the number corresponding to 10% of the total particle number).
Recently, cost reduction and resource conservation are highly demanded for the toner so that it is expected that elimination of the work for reuse from the cost reduction viewpoint and recycling instead of abandonment from the resource conservation viewpoint will be demanded in the future. Therefore, a toner without the need of the elimination work (or toner with a simplified elimination) has about a 5 xcexcm to 10 xcexcm volume average particle size, and the fine powdery toner with a less than 5 xcexcm particle size accounts for about as much as 60 to 80% of the whole.
When such a small particle size toner is used, the offset generation ratio is increased drastically compared not only with a toner with an ordinary particle size but also with a small particle size toner with the irregularity adjustment (toner with about a 5 xcexcm to 10 xcexcm volume average particle size, with the irregularity adjustment).
The relationship is shown in the table 1.
In this experiment, the number of paper passage is counted until the offset toner exceeding the tolerance of the cleaning member pressured against the fixing roller at the time of fixing a solid image on an ordinary A4 plain paper (with a good surface evenness) and a recycled paper (with a poor surface evenness). That is, a condition with a larger paper passage number represents a smaller offset.
Conventionally, in order to prevent the offset in the fixing roller, in addition to the configuration of providing the offset preventing layer, a configuration for eliminating the toner adhered on the fixing roller surface using a cleaning roller or a cleaning pad can be presented. However, according to the configuration, it is difficult to eliminate the toner following the offset amount when the small particle size toner is used. This is because the interval until the elimination amount by the cleaning member becomes shorter when the small particle size toner is used with a large amount of offset generated since the toner scraping amount is limited. Therefore, a problem is involved in that replacement of the member is needed frequently so as to increase the burden in the maintenance work.
The web taking out amount is set variously according to the use condition in the image formation apparatus, however, the taking out amount per unit time is small. That is, since the web installation space and the length of the replacement time are limited, it is not preferable to have a large taking out amount at one time, and thus, the taking amount per unit time is set to be about 0.5 to 2 mm in general.
When the entire web is consumed and taken up, it will be replaced. Therefore, a larger replacement interval is advantageous in terms of the maintenance cost. In the case of a device which has about a 100 sheets/1 minute image output ability, in general, the web operation interval is about 1 time (operation amount 2 mm) per 10 to 20 times (sheets) of the number of image formations, and the maintenance interval is per 300,000 sheets or more. With a longer web total extension distance, the number of winding at the time of being wound on the roller is increased as well.
With an increased number of winding, since the web is a non-woven fabric using a fiber, a gap is generated frequently between the adjacent non-woven fabrics so that the gap is increased between the non-woven fabrics according to the winding operation, and thus the winding loosening can easily be generated thereby.
FIGS. 7A, 7B and 7C are diagrams which show the phenomenon. As shown in FIG. 7A, when the number of winding is increased, the gap is generated frequently between the non-woven fabrics so that even when the taking up side roller shown by the mark A is rotated, only the gap between the non-woven fabrics is squeezed as shown in FIGS. 7B and 7C without movement of the web itself. As a result, the same surface of the web (part shown by the mark P in FIGS. 7A, 7B and C) is contacted on the fixing roller surface for a long time so that not only the offset toner or the foreign substance cannot be wiped off but also there is a risk of re-adhesion of the pollutant once wiped off onto the fixing roller surface due to inability of supporting the pollutant by the web itself.
Conventionally, as a configuration for preventing winding loosening of the web, the configuration provided with a member which pressures the surface of the taken out web has been proposed (for example, the Japanese Patent Application Laid-Open No. 63-50879). Moreover, the Japanese Patent Application Laid-Open No. 11-95600 discloses a configuration of contacting a braking member with a taking out shaft and pulling the braking member which applies the rotation load directly on the taking out side rotation shaft.
In the case of the former technique, not only damage of the web itself, in particular, the surface cannot be avoided but also the impregnated oil can be squeezed out since the web itself is in the squeezed state. Moreover, when the rotation load is increased, the web fiber becomes fluffy so as to fall of the fiber, and furthermore, when the winding diameter on the taking out roller side becomes large (the web extension distance is prolonged), the web roll shape collapses due to the pressuring force, and thus a problem of difficulty in the winding operation is involved.
In the case of the latter technique, since the rotation load is generated directly on the shaft, although the damage of the web itself as in the former case can be avoided, according to the configuration of providing the rotation load by a plate spring for executing winding fastening, since both the plate spring and the taking out shaft are rigid members, the contactor thereof has little engagement by deformation at the time of pressuring, even when the plate spring thickness is increased, the rotation load can be provided to the taking out shaft to about 100 gfxc2x7cm at most. That is, according to the rotation load by the plate spring, it is impossible to provide the necessary rotation load. Then, the present inventor has executed an experiment for finding out to what extent the rotation load is needed.
FIG. 8 is a graph which shows the rotation load value in the vertical axis and the slackening generation extent in the horizontal axis, with the results shown therein.
As it is apparent from the results shown in FIG. 8, with a longer web length, the gap between the non-woven fabrics is increased so as to easily generate loosening, and thus the pressuring force of the pressuring member should be increased gradually. According to the experiment, when a rotation load of 140 gfxc2x7cm is applied on the web taking out side roller, loosening is generated when the web is taken up by 16 m so that the web cannot be moved so as to generate cleaning failure. On the other hand, when a rotation load of 150 gfxc2x7cm is applied, the winding failure is not generated even when it is used to 30 m.
It is an object of the present invention to provide an image formation apparatus capable of certainly preventing generation of offset, the present invention is to provide a fixing device and an image formation apparatus comprising a configuration capable of certainly preventing generation of offset particularly when a small particle size toner is used. Furthermore, it is an object of the present invention to provide a fixing device and an image formation apparatus comprising a configuration capable of certainly preventing generation of offset particularly when a small particle size toner without particle size adjustment is used.
And, it is an object of the present invention to provide an image formation apparatus capable of certainly preventing generation of offset, particularly the present invention has been achieved in order to solve the problem of cleaning of a fixing roller, it is an another object of the present invention to provide a web differential gear and an image formation apparatus comprising a configuration capable of reducing loosening of the web without leading to damage on the web itself.
According to one aspect of the present invention, there is provided a fixing device which fixes the toner image on the sheet after transferring a toner image formed by visualizing an electrostatic latent image on a latent image supporting member onto a sheet, comprising, a web to be wound around on a pair of rollers, taken out from the taking out roller on one side to the taking up roller on the other side, with a part of the taken out part contacted with a member which heats and fuses the toner, and a controlling unit which controls the taking out amount of the taken out part according to the surface state of the sheet for supporting the toner image.
According to another aspect of the present invention, there is provided a web differential gear comprising, a taking out side rotation shaft with a web wound around for taking out the wound around web, a pressuring unit which pressures the taking out web against the surface of a subject to be cleaned, a taking up side rotation shaft for taking up the web pressured against the surface, and a load applying unit which directly applies a rotation load of 150 gfxc2x7cm or more on the taking out side rotation shaft of the web.
According to still another aspect of the present invention, an image formation apparatus comprises the fixing device.
According to still another aspect of the present invention, an image formation apparatus comprises the web differential gear.
Other objects and features of this invention will become understood from the following description with reference to the accompanying drawings.